Update: It appears that I was too quick in discounting Breyer as a possible flip. Evidence that he may even be likely to vote to overrule Harris can be found here .
A little inside baseball, for those of you who like this sort of thing.
US v. Harris was a 5-4 decision. Four justices have since retired, 2 from the majority, 2 from the dissent. They have been replaced by Kagan, Sotomayor, Alito and Roberts. I don't know offhand what Kagan and Sotomayor think of Blakely generally, but let's assume they are inclined to rule on the pro-Blakely-side of the ledger.
That would mean that either Alito or Roberts would need to vote on the pro-Blakely side for Harris to be overruled.
My impression is that Alito has no interest in extending Blakely, and he'd probably like to roll it back. He dissented in Southern Union and he was in the majority in Oregon v. Ice.
Roberts was the opposite. Majority in Southern Union and minority in Oregon v. Ice. So Roberts seems pretty pro-Blakely.
The three remaining members of the Harris majority are Scalia, Breyer and Kennedy. Breyer and Kennedy pretty strongly dislike Blakely. Scalia is passionately pro-Blakely, but clearly didn't believe it applied to mandatory minimums. But if anyone was going to flip, it would be Scalia.
Would stare decisis cause a dissenter from Harris to flip? Not impossible. Stevens was a dissenter in Almendarez-Torres and rumor has it, even after there was a majority to overule the case, he never voted to grant cert, since he felt it was decided and done. Stevens is gone, but the principle may carry some weight with the others.
Overall, I'd say the odds are not bad.